As I stated before if those two bands play in LA the same day I would go to see X-JAPAN (unless I win the contest …but what are the odds, right?? There are very good videos out there ;). Going to their (X JAPAN) concert has been my dream since I was 12, and back then it seemed impossible (until 2007, when they reunited and they made me very happy). I Know I will not be able to travel to San Francisco or somewhere else because of school, so even if I would like to go see both bands, I just would not be able to do so. I know this is a VAMPS forum, and it is probably kind of rude on my part to be saying this, but the things I take into account are:
1. I’ve seen VAMPS live before.
2. I think VAMPS will come back in the future, while I read somewhere (when they had just started to do lives again) that X JAPAN’s world tour would be their first and last. (? - if someone can verify this -?)

X JAPAN has not announced where in California they will start the tour, but I believe LA sounds like the most obvious choice (after all the X JAPAN-related events lately).
Still, I just REALLY hope (even though it clearly states "Oct. 1") the dates do not coincide or they start somewhere else, so I do not have to choose one over the other and be a bad fan!!